It is the goal of the current proposal to investigate the general hypothesis that resistance exercise stimulates muscle protein synthesis, but the magnitude of the response is determined by the availability of amino acids. A further goal of this proposal is to examine an important nutritional strategy for maximizing muscle protein anabolism by investigating the metabolic mechanisms by which amino acids and insulin contribute to net muscle protein synthesis. Rates of muscle protein synthesis and breakdown will be quantified in healthy human volunteers following resistance exercise by utilizing a new model that involves the use of stable isotopically labeled amino acid tracers. This unique methodology makes use of arteriovenous differences and muscle free intracellular amino acid enrichments and concentrations to directly determine in vivo rates of muscle protein synthesis and breakdown. Amino acid delivery to the leg and amino acid transport into and out of the muscle will also be determined by this model. Additionally, the model-derived values will be confirmed by the fundamentally independent measurement of protein synthesis by determining the fractional synthetic rate of muscle protein from the precursor-product method. A novel approach to directly determine fractional breakdown rate of muscle protein will also be utilized. The effect of increasing intracellular amino acid availability on muscle protein synthesis and breakdown will be assessed by providing amino acids following exercise and comparing the response to that without amino acids, both preceding and following 16 weeks of resistance exercise training. Protein synthesis should be increased by increasing amino acid availability in both the trained and untrained states, but by different mechanisms. Additionally, the hypothesis that the effect of non-protein substrates on muscle protein synthesis and breakdown is due to the stimulatory effect of insulin, rather than from an effect of the excess energy provided by the substrates will be tested. Following resistance exercise, the response of protein synthesis and breakdown to amino acid ingestion will be compared to ingestion of amino acids plus carbohydrates. We will then test the response to an amino acid ingestion plus local infusion of insulin to match blood insulin levels seen during the amino acid/carbohydrate ingestion. We will also examine the effect of the energy provided by the supplement on muscle protein metabolism following exercise by assessing the response of muscle protein metabolism to the ingestion of isocaloric amounts of amino acids, carbohydrates or lipids following resistance exercise. These studies will provide insight into the metabolic mechanisms for the regulation of muscle protein synthesis and breakdown following exercise, as well as provide practical information for optimizing muscle protein anabolism in exercising individuals. Aside from the obvious benefits to healthy, exercising individuals, information on increased muscle protein anabolism would be valuable for a variety of populations in which increasing muscle size and strength are important for overall health. These groups might include the elderly or those in rehabilitation from injuries, illnesses and burns.